Shrouded impellers are used in place of open impellers in a number of applications, including high blade loading conditions, in those situations in which there are requirements for reduced seal flow losses, requirements for high natural frequencies of the blades and certain high speed conditions where an attached shroud can reduce centrifugal stresses. A typical application for a shrouded impeller is in boiler feed water pumps.
Typical conventional methods of construction of shrouded impellers are (1) as a one-piece casting, with finish machining of the bore outside diameters and faces, and hand polishing of the inside of passageways; (2) casting or forging separately a hub and shroud, then machining them to shape, machining or forging vanes, welding the hub and shroud to the vanes, stress relieving the welded assembly, finish machining the bore and outside surfaces, and hand polishing the passageways, or, (3) for the greatest strength in conventional impellers, forging an oversize hub including vane profiles and forging a shroud, CNC or profiling machining passageways between vanes so that the vanes and hub are integral, CNC or profile machining a shroud, welding the shroud and vanes and hub together at the intersection of the vanes and the shroud, stress relieving the welded assembly, finish machining the bore and outside surfaces, and hand polishing the welded portion of the passageways.
In the cast impeller, the surfaces defining the passageways are likely to be pitted and rough in parts, simply by the nature of the casting process. Furthermore, the cast impeller is not as strong as a forged impeller. In both the second and third methods, the welds pose problems both in polishing and strength.
One of the objects of this invention is to provide a monolithic shrouded impeller of superior strength and efficiency as compared with shrouded impellers used heretofore.
Another object is to provide a method of making such an impeller.
Other objects will become apparent to those skilled in the art in the light of the following description and accompanying drawing.